Over the decades there has come to be a sort of implicit consensus that pet ownership had benefits for human health. That is, it seemed that these correlations weren’t coincidental. In a 2005 literature review in the journal BMJ, a team of clinicians concluded it’s likely that “pet ownership itself is the primary cause of the reported benefits,” since “no studies have found significant social or economic differences between people who do or do not have pets that would adequately explain [these] differences in health.”

It is with sincere regret that I report that this week’s RAND cat-health study did exactly that. The cat owners appeared healthier than people without pets, but the difference went away when the researchers factored in that the cat owners were likely to be healthy for other reasons, mostly bearing on socioeconomic status.

Even the researchers didn’t want to find what they found. Layla Parast, a Harvard-trained biostatician who does health analyses at the RAND corporation, approached the study with her colleagues fully expecting to see health benefits. The group “loves pets,” Parast insisted, as I probed for any evidence of anti-pet bias. She personally grew up with a dog and a cat, she claims.

“It was definitely our hypothesis that we’d find benefits. We assumed it would be a very straightforward thing to show.”

But no. The findings rather turned her understanding of the subject into a tangle of uncertainty. On Monday, the headline on RAND’s site was, “Largest-Ever Study of Pets and Kids’ Health Finds No Link; Findings Dispute Widely Held Beliefs About Positive Effects of Pet Ownership.”

Parast has already experienced social repercussions. “I've talked to a lot of friends of mine whose reaction was like yours and mine: No! This can't be true. What kind of ‘science’ are you doing?”

I imagine the genuine consternation on their faces matching that on mine. I’m not convinced that there is no link between pet ownership and health, and frankly neither is Parast. But it is worth looking at what kind of science she is doing, because it’s critical to how we understand health risks and benefits in much of life.

In this study, for example, the researchers looked at data from more than 5,000 households and analyzed children’s health by various measures, and then compared children in households without cats to children in those with cats. The kids with cats tended to have “better general health,” were less likely to have parents with concerns about their mood, behavior, or learning abilities, and were more likely to be physically active and deemed “obedient.”

This is all entirely consistent with what would be expected. What wasn’t expected was that once the researchers controlled for confounding variables, they had to report “no evidence for a beneficial effect of pet ownership for child health.” The pets didn’t make people healthier.

(Note that the researchers refer not simply to cats, but to “pets.” That is because the analysis studied the effects of owning cats and/or owning dogs. The findings were the same for both cats and dogs. Kids in dog-owning households tended to be healthier than those in pet-less households, but there was no evidence that dogs could account for that health difference. While I’m open to the possibility that this is true for cats, I’m not able to even comprehend it for dogs. So here I focus on cats.)

So if it’s not the pets themselves that are causing these health differences between pet-owning and non-pet-owning homes, what is?

Parast specializes in studying the sorts of health behaviors that aren’t amenable to randomized controlled trials. For example, smoking. It would be unethical to assign people to smoke cigarettes for 20 years and see if they developed emphysema. It would also be difficult to keep the test subjects blind. People tend to know when and what they are smoking.

The same is true of pet ownership. People who are given cats know that they have been given cats. There are no placebo cats. So there are studies that compare people in the real world who have lived with and without pets. This is the more feasible and accurate approach to understanding the effects of pets on human health. But the constant and gnawing limitation is that it’s difficult—if not impossible—to prove what sorts of behaviors are actually causing better or worse health outcomes. It took decades just to confirm that smoking was actually causing the mid-20th-century surge in lung cancer.

So what Parast did was look at all the variables that come with owning a pet—like living in a larger home and having greater household income. To be a confounding variable, a factor has to be associated with both the likelihood of owning a pet and with the outcome—in this case, child’s health (mental or physical).

After distilling all of these factors out of the equation, it turned out that the cats (and dogs) themselves were not associated with better health. Pet ownership is more of a signifier of the sort of life that leads to better health, not the driver of that better health. “We’re not completely ruling out that pet ownership leads to good health,” said Parast. “We’re just saying you need to step back and see that people who own pets are different from people who don’t in a whole lot of ways.”

The study can’t rule it out because it had a serious limitation. That’s what gives me hope, and Parast, too. It was a cross-sectional study, meaning it only studied people at a single point in time. (e.g., Does your house have a dog? Does your child have asthma?)

To really understand this relationship, a study would need to follow kids over time and see if they grew up healthier. This remains to be done on a large scale.

“I think there are many other positive benefits to owning a pet besides thinking that it will improve your health,” said Parast. “Obviously having a pet brings joy and companionship and a multitude of other things.”

I noted that those things are associated with mental health and physical health.

“Right,” she said. “But we don’t have measures of long-term outcomes to test that. I’ve heard people say that having a pet teaches responsibility, which is hard to measure. And if you really wanted to measure it, you’d test something like, 10 years later did this kid grow up to be someone who can hold a job?”

That sort of cohort study could also help understand how pet ownership may lead to better health. The idea is that, given this sort of longitudinal data over time, researchers could compare pet owners and non-owners, and then for example, five years later look at differences in physical activity among both groups. Then some years after that, researchers could look at health outcomes, and they could determine if maybe that physical activity is a mediating factor between pet ownership and health, meaning that essentially owning a dog does make people healthier.

“That would be great,” she said. “I mean, gosh, I hope that would find something. It would be great to have a reason to hand out cuddly puppies to everyone who needs better health. I would be completely in favor of that. But there’s no scientific evidence right now that shows that.”